The present invention relates to a method and device for, when data are to be recorded and reproduced onto and from an optical recording medium such as an optical card, performing tracking a light beam emitted from a recording/reproducing optical head relative to a desired data recording track of the medium.
Optical information recording and reproducing apparatuses are known which record and reproduce information onto and from a card-like optical recording medium (hereinafter referred to as an optical card) by moving the optical card relative to an optical head substantially at right angle with the optical axis of a laser beam emitted from the head. With the developments and increasing use of computers etc., a wide use of the optical card has been strongly hoped for in recent years because it is highly portable and safe and yet provides a relatively large storage capacity for its small size, and a variety of applications of the optical card have been proposed.
A typical structure of the optical card is shown in FIGS. 5A and 5B, of which FIG. 5A is a plan view of the known optical card 11 and FIG. 5B shows in enlarged scale a part (section "A") of the optical card 11 of FIG. 5A. In FIG. 5B, reference numeral 12 denotes a recording/reproducing area, 13 denotes guide tracks, and 14 denotes data recording tracks each provided between the guide tracks 13. On the recording/reproducing area 12 of the optical card 11, there are provided a multiplicity of guide tracks 13 and data recording tracks 14 in parallel relation to one another. The recording/reproducing area 12 includes a recording layer that is for example made of silver chloride photographic material as its base material. By irradiating a laser light spot of a suitable energy level from an optical head onto the recording layer, optical information units called "pits" 15 are formed or recorded in the data recording track 14. The position of the irradiated laser light spot on the recording layer is variable by moving the optical card 11 relative to the optical head in the X-axis direction (direction parallel to the length of the data and guide tracks 13 and 14 of the optical card 11), so that a series of pits can be formed in a desired arrangement corresponding to desired digital information. Thus, recording and reproduction of desired digital information are performed by writing and reading a row of the pits to and from the recording layer of the optical card 11. The guide tracks 13, and non-recorded portions of the data recording tracks 14 (i.e., portions having no data pit 15 formed therein) have different light reflecting characteristics; for example, the guide tracks 13 have a lower reflectivity than the unrecorded data recording track portions.
In order to form pit rows in the data recording track 14 of the optical card 11, such an approach is generally employed which uses a drive mechanism such as a linear motor to move the optical card 11 relative to the optical head. However, due to a limited operational accuracy of the drive mechanism, this prior approach can not reliably avoid occurrence of mechanical position errors, due to which pits can not be formed accurately in the central part of the data recording track 14 located between the guide tracks 13. This presents the significant problem that desired information can not be recorded accurately. In order to avoid this problem, it is absolutely necessary to perform the pit recording with the laser beam spot positioned in the central part of the recording track 14 precisely between two adjacent guide tracks 13. Reproduction of the recorded pits must also be performed with the laser beam spot accurately positioned in the central part of the recording track 14. To this end, automatic tracking control (often abbreviated "AT control") has been so far employed in an attempt to always position the laser beam spot at an optimum position while constantly compensating for any mechanical position errors caused.
The automatic tracking control has been performed so far in accordance with the so-called "three-beam method" for both recording and reproduction. According to the three-beam method, three laser beams spaced apart from each other by predetermined distances are irradiated from the optical head in such a manner that the central (main) laser beam corresponds to the data recording track 14 as a recording/reproducing beam and the two other (auxiliary) laser beams on both sides of the central beam correspond to the guide tracks 13 on both sides of the data recording track 14 as tracking beams. Namely, the three-beam method measures the respective reflected lights of the two auxiliary laser beams from the optical card 11 so as to servo-control the irradiated beam spot positions in such a manner that the tracking beams accurately correspond to the guide tracks 13 in predetermined positional relations thereto and thus the central main beam is allowed to be always accurately positioned in a predetermined central part of the data recording track 14. Further, it is necessary to have the laser light beams constantly stably focused on the recording layer of the optical card 11, and automatic focusing control has also conventionally been performed for this purpose.
The above-mentioned automatic tracking and focusing control operations are performed by minutely driving the objective lens of the optical head, via electromagnetic force applied via a tracking coil and a focusing coil, respectively, in the Y-axis direction (i.e., direction transverse to the data recording and guide tracks of the optical card 11) and in the Z-axis direction (i.e., direction perpendicular to the recording/reproducing surface of the optical card 11). The objective lens operates to focus the laser beam emitted from the optical head onto the recording layer of the optical card 11 so as to form a focused light spot (three light spots in the case where the above-mentioned three-beam method is employed) on the recording layer.
The automatic tracking control according to the three-beam method will be described below in greater detail. According to the three-beam method, the main beam 30 and auxiliary beams 31, 32 are emitted from the optical head so that the beams are irradiated onto the optical card 11 in predetermined positional relations to each other, as shown in FIG. 6A. In such a manner that about half portions of the auxiliary beams 31 and 32 are accurately irradiated onto the two guide tracks 13 on both sides of the data recording track 14, the respective positions of the irradiated beams from the optical head are controlled minutely by means of the above-mentioned tracking coil while the optical card 11 are moved relative to the optical head in the direction parallel to the length of the recording track 14 (i.e., X-axis direction). In this case, signals indicative of reflected light quantities (reflected light signals) of the auxiliary beams 31 and 32 are differentially amplified, and then servo control is performed so that the differentially amplified output becomes null. In recording data, data pits are formed in the data recording track 14 by increasing the light intensity of the main beam 30 for each predetermined data recording point while performing the tracking control in the above-mentioned manner.
Similarly, for reproduction of the recorded data according to the three-beam method, the main laser beam 30 is irradiated onto the data recording track 14 while performing the tracking control by radiating the three beams in such a manner that about half portions of the auxiliary beams 31 and 32 are accurately irradiated onto the two guide tracks 13. Specifically, the recorded data are reproduced by transforming detected variations in intensity of reflected light from the data recording track 14 into electric current variations. The main beam 30 used for reproducing data is set to a lower intensity than that used for recording data; typically, the reproducing beam intensity is about one tenth of the recording beam intensity.
Although the three-beam method is a highly stable method that is often used in cases where the tracking control is performed by irradiating the auxiliary beams on both sides of a designated data recording track as in reproduction of a CD (Compact Disk), this method presents the inconvenience that some data are undesirably recorded off the center of the recording track if the guide tracks on both sides of the recording track have non-uniform widths and light reflectivities and/or if the light intensity of the two auxiliary beams do not balance.
That is, if the guide tracks 13 on both sides of the recording track 14 have uniform widths and light reflectivities and the light intensity of the two auxiliary beams is equal, the two auxiliary beams 31 and 32 are positioned at locations such that respective halves of the auxiliary beams 31 and 32 can be accurately irradiated onto the two guide tracks 13, i.e., exactly halves of the two auxiliary beams 31 and 32 can be irradiated onto the respective guide tracks 13, as shown in FIG. 6A. Thus, the main beam 30 can be irradiated accurately onto the central part of the data recording track 14, so that center line 14c of a row of recorded data pits in the track 14 lies exactly in the middle between the two guide tracks 13 without any positional deviation.
However, when the reflectivity of one of the guide tracks 13 (e.g., guide track 13a) has become higher than the other guide track 13 (e.g., guide track 13b), the automatic tracking is performed in such a manner that more than half of the auxiliary beam spot 31 is irradiated onto the higher-reflectivity guide track 13a and less than half of the auxiliary beam spot 32 is irradiated onto the lower-reflectivity guide track 13b as shown in FIG. 6B so that the respective reflection intensity of the two auxiliary beams 31 and 32 becomes equal. Consequently, the main beam spot 30 is placed off the center of the data recording track 14 toward one guide track 13a, and the center line 14c of a row of recorded data pits in the track 14 deviates from the center of the recording track 14, as shown in FIG. 6B.
Conversely, when the reflectivity of the guide track 13a has become lower than the other guide track 13b, the tracking is performed in such a manner that the irradiated spot of the main beam 30 is positioned off the center of the recording track 14 toward the guide track 13b so that the center line 14c of a row of recorded data pits in the track 14 deviates from the center of the recording track 14 toward the guide track 13b.
Further, when the light intensity of one of the auxiliary beams (e.g., auxiliary beam 31) has become higher than the other auxiliary beam (e.g., auxiliary beam 32), the automatic tracking is performed in such a manner that more than half of the auxiliary beam spot 31 is irradiated onto the guide track 13a and less than half of the auxiliary beam spot 32 is irradiated onto the guide track 13b as shown in FIG. 6C so that the respective reflection intensity of the two auxiliary beams 31 and 32 balances. Consequently, the main beam spot 30 is placed off the center of the data recording track 14 toward one guide track 13a, and the center line 14c of a row of recorded data pits in the track 14 deviates from the center of the recording track 14 toward the guide track 13a, as shown in FIG. 6C.
Conversely, when the reflectivity of the auxiliary beam 31 has become lower than the other beam 32, the tracking is performed in such a manner that the irradiated spot of the main beam 30 is positioned off the center of the recording track 14 toward the guide track 13b, with the result that the center line 14c of a row of recorded data pits in the track 14 deviates from the center of the recording track 14 toward the guide track 13b.
The positional deviation of the center line 14c of a recorded data pit row does not present significant inconveniences as long as data reproduction is performed exactly in the same conditions as in data recording. However, because conditions in data reproduction usually differ from those in data recording, there would occur the problem that levels of reproduced data signals are undesirably lowered.
Assume a case where the light intensities of the two auxiliary beams used in data recording were unbalanced, and thus data pits were recorded off the center of the data recording track 14 toward one of the guide tracks 13a, causing positional deviation of the center line 14c of the recorded data pit row as shown in FIG. 7. If, in this case, the light intensity of the two auxiliary beams to be used in data reproduction do balance or are unbalanced in an opposite manner to the unbalanced condition in the recording, the automatic tracking is performed in such a manner to position the reproducing main beam 30 off the center line 14c of the recorded data pit row as shown in FIG. 7. Due to such a positional deviation between the data pits 15 and the main beam 30, variations in the reflected light intensity would be reduced to present reduced levels of reproduced data signals, so that the C/N (Carrier-to-Noise) ratio is reduced. This results in increased errors in the reproduced data. Further, even when the center line 14c of the recorded data pit row is coincident with the center of the recording track 14, similar problems occur in the event that the two auxiliary beams to be used in data reproduction have unequal light intensity.